1. Field of the Invention
This invention relates to a drill bit, and more particularly to a drill bit having a pair of cutting edges spaced from each other with the rotational axis of the drill bit intervening therebetween.
2. Description of the Prior Art
In U.S. Pat. No. 4,373,839 (Negishi et al.), there is disclosed a drill bit having a pair of cutting edges which are opposed to each other with the rotational axis of the drill bit intervening therebetween. That is, a slot or a non-cutting portion which does not serve for cutting is formed between the cutting edges at a portion including the rotational axis of the drill bit. The portions of the respective cutting edges nearest to the rotational axis are symmetrically disposed with respect to the rotational axis of the drill bit. That is, the portions of the respective cutting edges nearest to the rotational axis of the drill bit (Each of these portions will be referred to as "the innermost point of the cutting edge" in this specification.) are diametrically opposed to each other (In other words, the innermost points of the cutting edges are on a line passing through the rotational axis of the drill bit as viewed in the axial direction of the drill bit.) and are equally spaced from the rotational axis by a predetermined distance which is generally from 0.l mm to 1.25 mm.
The U.S. patent identified above says that the slot between the cutting edges serves to reduce the thrust load acting on the drill bit during drilling operation, to prevent melt-adhesion of cuttings to the cutting edge and to prevent shaking of the drill bit. Further it is said that, though a cylindrical core (residual part of the workpiece) is first formed on the bottom of the hole being drilled by virtue of the slot between the cutting edges, the core is twisted off during the drilling operation before it grows to an unacceptable height since the width of the slot is so defined.
However, according to our investigation, the drill bit is unsatisfactory in that cuttings and/or the twisted-off core are apt to clog the slot between the cutting edges to adversely affect the drilling operation or to break the cutting edge.